update check in is_array_container_type to an assert

Author: alexfikl

arraycontext/container/__init__.py

@@ -173,12 +173,11 @@ def is_array_container_type(cls: type) -> bool:
         function will say that :class:`numpy.ndarray` is an array container
         type, only object arrays *actually are* array containers.
     """
-    # NOTE: `is_array_container_type` is used in `dataclass_array_container`
-    # where it looks at field types. The fields can contain typing information
-    # like `typing.Optional[MyType]`, which does not have an `__mro__` and messes
-    # up `singledispatch.dispatch`, so we check for that first
+    assert isinstance(cls, type), \
+            f"must pass a type, not an instance: '{cls!r}'"
+    assert hasattr(cls, "__mro__"), "'cls' has no attribute '__mro__': "
 
-    return hasattr(cls, "__mro__") and (
+    return (
             cls is ArrayContainer
             or (serialize_container.dispatch(cls)
                 is not serialize_container.__wrapped__))  # type:ignore[attr-defined]

test/test_utils.py

@@ -59,6 +59,7 @@ class ArrayContainerWithStringTypes:
         y: "np.ndarray"
 
     with pytest.raises(AssertionError):
+        # NOTE: cannot have string annotations in container
         dataclass_array_container(ArrayContainerWithStringTypes)
 
     # }}}
@@ -70,8 +71,9 @@ class ArrayContainerWithOptional:
         x: np.ndarray
         y: Optional[np.ndarray]
 
-    cls = dataclass_array_container(ArrayContainerWithOptional)
-    assert cls is not None
+    with pytest.raises(AssertionError):
+        # NOTE: cannot have wrapped annotations (here by `Optional`)
+        dataclass_array_container(ArrayContainerWithOptional)
 
     # }}}